1. Field of the Invention
The present invention relates to an improvement in the production of porous separation membranes from polyolefins such as polypropylene and polyethylene via thermally induced phase separation.
2. Description of the Prior Art
Hitherto, a wide variety of methods have been developed and suggested for preparing microporous membranes and films.
For example, U.S. Pat. No. 4,539,256 to Shipman discloses a microporous sheet material, a method of making the same, and articles of manufacture according to the method. This patent teaches a method comprising the steps of melt blending a semicrystalline thermoplastic polymer with a particular type of blending compounds, forming a shaped article of the melt blend, cooling the shaped article to cause phase separation between the thermoplastic polymer and the compounds, and orienting the article to provide a network of interconnected micropores.
U.S. Pat. No. 4,247,498 to Castro discloses the preparation of microporous polymer products having a homogeneous, three-dimensional cellular structure which comprises heating a thermoplastic polymer with an appropriate compatible liquid to form a homogeneous solution, cooling the resulting solution to initiate liquid-liquid phase separation, and subsequently continuing said cooling to solidify the polymer, and to remove the liquid to form a microporous material.
The foregoing prior art techniques have not, however, been commercially succeeded probably due to the lack of economic feasibility of the techniques. Thus, there has existed a need for facile processes which allow economically viable production of porous polymer membranes and films.